Add content scale queries

This adds glfwGetWindowContentScale and glfwGetMonitorContentScale for
querying the recommended drawing scale factor for DPI-aware rendering.

Parts of this patch are based on code by @ferreiradaselva.

Fixes #235.
Fixes #439.
Fixes #677.
Fixes #845.
Fixes #898.

README.md

@@ -136,6 +136,8 @@ information on what to include when reporting a bug.
   gamepad mapping (#900)
 - Added `glfwGetGamepadState` function, `GLFW_GAMEPAD_*` and `GLFWgamepadstate`
   for retrieving gamepad input state (#900)
+- Added `glfwGetWindowContentScale` and `glfwGetMonitorContentScale` for
+  DPI-aware rendering (#235,#439,#677,#845,#898)
 - Added `glfwRequestWindowAttention` function for requesting attention from the
   user (#732,#988)
 - Added `glfwGetKeyScancode` function that allows retrieving platform dependent

docs/monitor.dox

@@ -131,17 +131,34 @@ current _resolution_, i.e. the width and height of its current
 [video mode](@ref monitor_modes).
 
 @code
-int widthMM, heightMM;
+int width_mm, height_mm;
-glfwGetMonitorPhysicalSize(monitor, &widthMM, &heightMM);
+glfwGetMonitorPhysicalSize(monitor, &width_mm, &height_mm);
 @endcode
 
-This can, for example, be used together with the current video mode to calculate
+While this can be used to calculate the raw DPI of a monitor, this is often not
-the DPI of a monitor. 
+useful.  Instead use the [monitor content scale](@ref monitor_scale) and
+[window content scale](@ref window_scale) to scale your content.
+
+
+@subsection monitor_scale Content scale
+
+The content scale for a monitor can be retrieved with @ref
+glfwGetMonitorContentScale.
 
 @code
-const double dpi = mode->width / (widthMM / 25.4);
+float xscale, yscale;
+glfwGetMonitorContentScale(monitor, &xscale, &yscale);
 @endcode
 
+The content scale is the ratio between the current DPI and the platform's
+default DPI.  If you scale all pixel dimensions by this scale then your content
+should appear at an appropriate size.  This is especially important for text and
+any UI elements.
+
+The content scale may depend on both the monitor resolution and pixel density
+and on user settings.  It may be very different from the raw DPI calculated from
+the physical size and current resolution.    
+
 
 @subsection monitor_pos Virtual position
 

docs/news.dox

@@ -58,6 +58,15 @@ windows with @ref glfwSetWindowAttrib.
 @see @ref window_attribs
 
 
+@subsection news_33_contentscale Content scale queries for DPI-aware rendering
+
+GLFW now supports querying the window and monitor content scale, i.e. the ratio
+between the current DPI and the platform's default DPI, with @ref
+glfwGetWindowContentScale and @ref glfwGetMonitorContentScale.
+
+@see @ref window_scale
+
+
 @subsection news_33_inithint Support for initialization hints
 
 GLFW now supports setting library initialization hints with @ref glfwInitHint

docs/window.dox

@@ -663,6 +663,26 @@ The size of a framebuffer may change independently of the size of a window, for
 example if the window is dragged between a regular monitor and a high-DPI one.
 
 
+@subsection window_scale Window content scale
+
+The content scale for a window can be retrieved with @ref
+glfwGetWindowContentScale.
+
+@code
+float xscale, yscale;
+glfwGetWindowContentScale(window, &xscale, &yscale);
+@endcode
+
+The content scale of a window is the ratio between the current DPI and the
+platform's default DPI.  If you scale all pixel dimensions by this scale then
+your content should appear at an appropriate size.  This is especially important
+for text and any UI elements.
+
+On systems where each monitors can have its own content scale, the window
+content scale will depend on which monitor the system considers the window to be
+on.
+
+
 @subsection window_sizelimits Window size limits
 
 The minimum and maximum size of the client area of a windowed mode window can be

include/GLFW/glfw3.h

@@ -1926,6 +1926,36 @@ GLFWAPI void glfwGetMonitorPos(GLFWmonitor* monitor, int* xpos, int* ypos);
  */
 GLFWAPI void glfwGetMonitorPhysicalSize(GLFWmonitor* monitor, int* widthMM, int* heightMM);
 
+/*! @brief Retrieves the content scale for the specified monitor.
+ *
+ *  This function retrieves the content scale for the specified monitor.  The
+ *  content scale is the ratio between the current DPI and the platform's
+ *  default DPI.  If you scale all pixel dimensions by this scale then your
+ *  content should appear at an appropriate size.  This is especially important
+ *  for text and any UI elements.
+ *
+ *  The content scale may depend on both the monitor resolution and pixel
+ *  density and on user settings.  It may be very different from the raw DPI
+ *  calculated from the physical size and current resolution.
+ *
+ *  @param[in] monitor The monitor to query.
+ *  @param[out] xscale Where to store the x-axis content scale, or `NULL`.
+ *  @param[out] yscale Where to store the y-axis content scale, or `NULL`.
+ *
+ *  @errors Possible errors include @ref GLFW_NOT_INITIALIZED and @ref
+ *  GLFW_PLATFORM_ERROR.
+ *
+ *  @thread_safety This function must only be called from the main thread.
+ *
+ *  @sa @ref monitor_scale
+ *  @sa @ref glfwGetWindowContentScale
+ *
+ *  @since Added in version 3.3.
+ *
+ *  @ingroup monitor
+ */
+GLFWAPI void glfwGetMonitorContentScale(GLFWmonitor* monitor, float* xscale, float* yscale);
+
 /*! @brief Returns the name of the specified monitor.
  *
  *  This function returns a human-readable name, encoded as UTF-8, of the
@@ -2774,6 +2804,36 @@ GLFWAPI void glfwGetFramebufferSize(GLFWwindow* window, int* width, int* height)
  */
 GLFWAPI void glfwGetWindowFrameSize(GLFWwindow* window, int* left, int* top, int* right, int* bottom);
 
+/*! @brief Retrieves the content scale for the specified window.
+ *
+ *  This function retrieves the content scale for the specified window.  The
+ *  content scale is the ratio between the current DPI and the platform's
+ *  default DPI.  If you scale all pixel dimensions by this scale then your
+ *  content should appear at an appropriate size.  This is especially important
+ *  for text and any UI elements.
+ *
+ *  On systems where each monitors can have its own content scale, the window
+ *  content scale will depend on which monitor the system considers the window
+ *  to be on.
+ *
+ *  @param[in] window The window to query.
+ *  @param[out] xscale Where to store the x-axis content scale, or `NULL`.
+ *  @param[out] yscale Where to store the y-axis content scale, or `NULL`.
+ *
+ *  @errors Possible errors include @ref GLFW_NOT_INITIALIZED and @ref
+ *  GLFW_PLATFORM_ERROR.
+ *
+ *  @thread_safety This function must only be called from the main thread.
+ *
+ *  @sa @ref window_scale
+ *  @sa @ref glfwGetMonitorContentScale
+ *
+ *  @since Added in version 3.3.
+ *
+ *  @ingroup window
+ */
+GLFWAPI void glfwGetWindowContentScale(GLFWwindow* window, float* xscale, float* yscale);
+
 /*! @brief Iconifies the specified window.
  *
  *  This function iconifies (minimizes) the specified window if it was

src/cocoa_monitor.m

@@ -229,6 +229,9 @@ void _glfwPollMonitorsNS(void)
     displays = calloc(displayCount, sizeof(CGDirectDisplayID));
     CGGetOnlineDisplayList(displayCount, displays, &displayCount);
 
+    for (i = 0;  i < _glfw.monitorCount;  i++)
+        _glfw.monitors[i]->ns.screen = nil;
+
     disconnectedCount = _glfw.monitorCount;
     if (disconnectedCount)
     {
@@ -371,6 +374,48 @@ void _glfwPlatformGetMonitorPos(_GLFWmonitor* monitor, int* xpos, int* ypos)
         *ypos = (int) bounds.origin.y;
 }
 
+void _glfwPlatformGetMonitorContentScale(_GLFWmonitor* monitor,
+                                         float* xscale, float* yscale)
+{
+    if (!monitor->ns.screen)
+    {
+        NSUInteger i;
+        NSArray* screens = [NSScreen screens];
+
+        for (i = 0;  i < [screens count];  i++)
+        {
+            NSScreen* screen = [screens objectAtIndex:i];
+            NSNumber* displayID =
+                [[screen deviceDescription] objectForKey:@"NSScreenNumber"];
+
+            // HACK: Compare unit numbers instead of display IDs to work around
+            //       display replacement on machines with automatic graphics
+            //       switching
+            if (monitor->ns.unitNumber ==
+                CGDisplayUnitNumber([displayID unsignedIntValue]))
+            {
+                monitor->ns.screen = screen;
+                break;
+            }
+        }
+
+        if (i == [screens count])
+        {
+            _glfwInputError(GLFW_PLATFORM_ERROR,
+                            "Cocoa: Failed to find a screen for monitor");
+            return;
+        }
+    }
+
+    const NSRect points = [monitor->ns.screen frame];
+    const NSRect pixels = [monitor->ns.screen convertRectToBacking:points];
+
+    if (xscale)
+        *xscale = (float) (pixels.size.width / points.size.width);
+    if (yscale)
+        *yscale = (float) (pixels.size.height / points.size.height);
+}
+
 GLFWvidmode* _glfwPlatformGetVideoModes(_GLFWmonitor* monitor, int* count)
 {
     CFArrayRef modes;

src/cocoa_platform.h

@@ -135,6 +135,7 @@ typedef struct _GLFWmonitorNS
     CGDirectDisplayID   displayID;
     CGDisplayModeRef    previousMode;
     uint32_t            unitNumber;
+    id                  screen;
 
 } _GLFWmonitorNS;
 

src/cocoa_window.m

@@ -1288,6 +1288,18 @@ void _glfwPlatformGetWindowFrameSize(_GLFWwindow* window,
         *bottom = contentRect.origin.y - frameRect.origin.y;
 }
 
+void _glfwPlatformGetWindowContentScale(_GLFWwindow* window,
+                                        float* xscale, float* yscale)
+{
+    const NSRect points = [window->ns.view frame];
+    const NSRect pixels = [window->ns.view convertRectToBacking:points];
+
+    if (xscale)
+        *xscale = (float) (pixels.size.width / points.size.width);
+    if (yscale)
+        *yscale = (float) (pixels.size.height / points.size.height);
+}
+
 void _glfwPlatformIconifyWindow(_GLFWwindow* window)
 {
     [window->ns.object miniaturize:nil];

src/internal.h

@@ -641,6 +641,7 @@ const char* _glfwPlatformGetScancodeName(int scancode);
 int _glfwPlatformGetKeyScancode(int key);
 
 void _glfwPlatformGetMonitorPos(_GLFWmonitor* monitor, int* xpos, int* ypos);
+void _glfwPlatformGetMonitorContentScale(_GLFWmonitor* monitor, float* xscale, float* yscale);
 GLFWvidmode* _glfwPlatformGetVideoModes(_GLFWmonitor* monitor, int* count);
 void _glfwPlatformGetVideoMode(_GLFWmonitor* monitor, GLFWvidmode* mode);
 void _glfwPlatformGetGammaRamp(_GLFWmonitor* monitor, GLFWgammaramp* ramp);
@@ -670,6 +671,7 @@ void _glfwPlatformSetWindowSizeLimits(_GLFWwindow* window, int minwidth, int min
 void _glfwPlatformSetWindowAspectRatio(_GLFWwindow* window, int numer, int denom);
 void _glfwPlatformGetFramebufferSize(_GLFWwindow* window, int* width, int* height);
 void _glfwPlatformGetWindowFrameSize(_GLFWwindow* window, int* left, int* top, int* right, int* bottom);
+void _glfwPlatformGetWindowContentScale(_GLFWwindow* window, float* xscale, float* yscale);
 void _glfwPlatformIconifyWindow(_GLFWwindow* window);
 void _glfwPlatformRestoreWindow(_GLFWwindow* window);
 void _glfwPlatformMaximizeWindow(_GLFWwindow* window);

src/mir_monitor.c

@@ -88,6 +88,15 @@ void _glfwPlatformGetMonitorPos(_GLFWmonitor* monitor, int* xpos, int* ypos)
         *ypos = monitor->mir.y;
 }
 
+void _glfwPlatformGetMonitorContentScale(_GLFWmonitor* monitor,
+                                         float* xscale, float* yscale)
+{
+    if (xscale)
+        *xscale = 1.f;
+    if (yscale)
+        *yscale = 1.f;
+}
+
 static void FillInRGBBitsFromPixelFormat(GLFWvidmode* mode, const MirPixelFormat pf)
 {
     switch (pf)

src/mir_window.c

@@ -515,6 +515,15 @@ void _glfwPlatformGetWindowSize(_GLFWwindow* window, int* width, int* height)
         *height = window->mir.height;
 }
 
+void _glfwPlatformGetWindowContentScale(_GLFWwindow* window,
+                                        float* xscale, float* yscale)
+{
+    if (xscale)
+        *xscale = 1.f;
+    if (yscale)
+        *yscale = 1.f;
+}
+
 void _glfwPlatformIconifyWindow(_GLFWwindow* window)
 {
     MirWindowSpec* spec;

src/monitor.c

@@ -314,6 +314,21 @@ GLFWAPI void glfwGetMonitorPhysicalSize(GLFWmonitor* handle, int* widthMM, int*
         *heightMM = monitor->heightMM;
 }
 
+GLFWAPI void glfwGetMonitorContentScale(GLFWmonitor* handle,
+                                        float* xscale, float* yscale)
+{
+    _GLFWmonitor* monitor = (_GLFWmonitor*) handle;
+    assert(monitor != NULL);
+
+    if (xscale)
+        *xscale = 0.f;
+    if (yscale)
+        *yscale = 0.f;
+
+    _GLFW_REQUIRE_INIT();
+    _glfwPlatformGetMonitorContentScale(monitor, xscale, yscale);
+}
+
 GLFWAPI const char* glfwGetMonitorName(GLFWmonitor* handle)
 {
     _GLFWmonitor* monitor = (_GLFWmonitor*) handle;

src/null_monitor.c

@@ -36,6 +36,15 @@ void _glfwPlatformGetMonitorPos(_GLFWmonitor* monitor, int* xpos, int* ypos)
 {
 }
 
+void _glfwPlatformGetMonitorContentScale(_GLFWmonitor* monitor,
+                                         float* xscale, float* yscale)
+{
+    if (xscale)
+        *xscale = 1.f;
+    if (yscale)
+        *yscale = 1.f;
+}
+
 GLFWvidmode* _glfwPlatformGetVideoModes(_GLFWmonitor* monitor, int* found)
 {
     return NULL;

src/null_window.c

@@ -139,6 +139,15 @@ void _glfwPlatformGetWindowFrameSize(_GLFWwindow* window,
 {
 }
 
+void _glfwPlatformGetWindowContentScale(_GLFWwindow* window,
+                                        float* xscale, float* yscale)
+{
+    if (xscale)
+        *xscale = 1.f;
+    if (yscale)
+        *yscale = 1.f;
+}
+
 void _glfwPlatformIconifyWindow(_GLFWwindow* window)
 {
 }

src/win32_init.c

@@ -151,6 +151,8 @@ static GLFWbool loadLibraries(void)
     {
         _glfw.win32.shcore.SetProcessDpiAwareness_ = (PFN_SetProcessDpiAwareness)
             GetProcAddress(_glfw.win32.shcore.instance, "SetProcessDpiAwareness");
+        _glfw.win32.shcore.GetDpiForMonitor_ = (PFN_GetDpiForMonitor)
+            GetProcAddress(_glfw.win32.shcore.instance, "GetDpiForMonitor");
     }
 
     return GLFW_TRUE;

src/win32_monitor.c

@@ -60,6 +60,7 @@ static _GLFWmonitor* createMonitor(DISPLAY_DEVICEW* adapter,
                                    DISPLAY_DEVICEW* display)
 {
     _GLFWmonitor* monitor;
+    int widthMM, heightMM;
     char* name;
     HDC dc;
     DEVMODEW dm;
@@ -72,13 +73,26 @@ static _GLFWmonitor* createMonitor(DISPLAY_DEVICEW* adapter,
     if (!name)
         return NULL;
 
+    ZeroMemory(&dm, sizeof(dm));
+    dm.dmSize = sizeof(dm);
+    EnumDisplaySettingsW(adapter->DeviceName, ENUM_CURRENT_SETTINGS, &dm);
+
     dc = CreateDCW(L"DISPLAY", adapter->DeviceName, NULL, NULL);
 
-    monitor = _glfwAllocMonitor(name,
+    if (IsWindows8Point1OrGreater())
-                                GetDeviceCaps(dc, HORZSIZE),
+    {
-                                GetDeviceCaps(dc, VERTSIZE));
+        widthMM  = GetDeviceCaps(dc, HORZSIZE);
+        heightMM = GetDeviceCaps(dc, VERTSIZE);
+    }
+    else
+    {
+        widthMM  = (int) (dm.dmPelsWidth * 25.4f / GetDeviceCaps(dc, LOGPIXELSX));
+        heightMM = (int) (dm.dmPelsHeight * 25.4f / GetDeviceCaps(dc, LOGPIXELSY));
+    }
 
     DeleteDC(dc);
+
+    monitor = _glfwAllocMonitor(name, widthMM, heightMM);
     free(name);
 
     if (adapter->StateFlags & DISPLAY_DEVICE_MODESPRUNED)
@@ -101,10 +115,6 @@ static _GLFWmonitor* createMonitor(DISPLAY_DEVICEW* adapter,
                             NULL, NULL);
     }
 
-    ZeroMemory(&dm, sizeof(dm));
-    dm.dmSize = sizeof(dm);
-    EnumDisplaySettingsW(adapter->DeviceName, ENUM_CURRENT_SETTINGS, &dm);
-
     rect.left   = dm.dmPosition.x;
     rect.top    = dm.dmPosition.y;
     rect.right  = dm.dmPosition.x + dm.dmPelsWidth;
@@ -302,6 +312,26 @@ void _glfwRestoreVideoModeWin32(_GLFWmonitor* monitor)
     }
 }
 
+void _glfwGetMonitorContentScaleWin32(HMONITOR handle, float* xscale, float* yscale)
+{
+    UINT xdpi, ydpi;
+
+    if (IsWindows8Point1OrGreater())
+        GetDpiForMonitor(handle, MDT_EFFECTIVE_DPI, &xdpi, &ydpi);
+    else
+    {
+        const HDC dc = GetDC(NULL);
+        xdpi = GetDeviceCaps(dc, LOGPIXELSX);
+        ydpi = GetDeviceCaps(dc, LOGPIXELSY);
+        ReleaseDC(NULL, dc);
+    }
+
+    if (xscale)
+        *xscale = xdpi / 96.f;
+    if (yscale)
+        *yscale = ydpi / 96.f;
+}
+
 
 //////////////////////////////////////////////////////////////////////////
 //////                       GLFW platform API                      //////
@@ -324,6 +354,12 @@ void _glfwPlatformGetMonitorPos(_GLFWmonitor* monitor, int* xpos, int* ypos)
         *ypos = dm.dmPosition.y;
 }
 
+void _glfwPlatformGetMonitorContentScale(_GLFWmonitor* monitor,
+                                         float* xscale, float* yscale)
+{
+    _glfwGetMonitorContentScaleWin32(monitor->win32.handle, xscale, yscale);
+}
+
 GLFWvidmode* _glfwPlatformGetVideoModes(_GLFWmonitor* monitor, int* count)
 {
     int modeIndex = 0, size = 0;

src/win32_platform.h

@@ -135,6 +135,13 @@ typedef enum PROCESS_DPI_AWARENESS
     PROCESS_SYSTEM_DPI_AWARE = 1,
     PROCESS_PER_MONITOR_DPI_AWARE = 2
 } PROCESS_DPI_AWARENESS;
+typedef enum MONITOR_DPI_TYPE
+{
+    MDT_EFFECTIVE_DPI = 0,
+    MDT_ANGULAR_DPI = 1,
+    MDT_RAW_DPI = 2,
+    MDT_DEFAULT = MDT_EFFECTIVE_DPI
+} MONITOR_DPI_TYPE;
 #endif /*DPI_ENUMS_DECLARED*/
 
 // HACK: Define versionhelpers.h functions manually as MinGW lacks the header
@@ -216,7 +223,9 @@ typedef HRESULT(WINAPI * PFN_DwmEnableBlurBehindWindow)(HWND,const DWM_BLURBEHIN
 
 // shcore.dll function pointer typedefs
 typedef HRESULT (WINAPI * PFN_SetProcessDpiAwareness)(PROCESS_DPI_AWARENESS);
+typedef HRESULT (WINAPI * PFN_GetDpiForMonitor)(HMONITOR,MONITOR_DPI_TYPE,UINT*,UINT*);
 #define SetProcessDpiAwareness _glfw.win32.shcore.SetProcessDpiAwareness_
+#define GetDpiForMonitor _glfw.win32.shcore.GetDpiForMonitor_
 
 typedef VkFlags VkWin32SurfaceCreateFlagsKHR;
 
@@ -326,6 +335,7 @@ typedef struct _GLFWlibraryWin32
     struct {
         HINSTANCE                       instance;
         PFN_SetProcessDpiAwareness      SetProcessDpiAwareness_;
+        PFN_GetDpiForMonitor            GetDpiForMonitor_;
     } shcore;
 
 } _GLFWlibraryWin32;
@@ -395,4 +405,5 @@ void _glfwInitTimerWin32(void);
 void _glfwPollMonitorsWin32(void);
 GLFWbool _glfwSetVideoModeWin32(_GLFWmonitor* monitor, const GLFWvidmode* desired);
 void _glfwRestoreVideoModeWin32(_GLFWmonitor* monitor);
+void _glfwGetMonitorContentScaleWin32(HMONITOR handle, float* xscale, float* yscale);
 

src/win32_window.c

@@ -983,19 +983,6 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg,
             break;
         }
 
-        case WM_DPICHANGED:
-        {
-            RECT* rect = (RECT*) lParam;
-            SetWindowPos(window->win32.handle,
-                         HWND_TOP,
-                         rect->left,
-                         rect->top,
-                         rect->right - rect->left,
-                         rect->bottom - rect->top,
-                         SWP_NOACTIVATE | SWP_NOZORDER);
-            break;
-        }
-
         case WM_DROPFILES:
         {
             HDROP drop = (HDROP) wParam;
@@ -1415,6 +1402,14 @@ void _glfwPlatformGetWindowFrameSize(_GLFWwindow* window,
         *bottom = rect.bottom - height;
 }
 
+void _glfwPlatformGetWindowContentScale(_GLFWwindow* window,
+                                        float* xscale, float* yscale)
+{
+    const HANDLE handle = MonitorFromWindow(window->win32.handle,
+                                            MONITOR_DEFAULTTONEAREST);
+    _glfwGetMonitorContentScaleWin32(handle, xscale, yscale);
+}
+
 void _glfwPlatformIconifyWindow(_GLFWwindow* window)
 {
     ShowWindow(window->win32.handle, SW_MINIMIZE);

src/window.c

@@ -632,6 +632,21 @@ GLFWAPI void glfwGetWindowFrameSize(GLFWwindow* handle,
     _glfwPlatformGetWindowFrameSize(window, left, top, right, bottom);
 }
 
+GLFWAPI void glfwGetWindowContentScale(GLFWwindow* handle,
+                                       float* xscale, float* yscale)
+{
+    _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
+    if (xscale)
+        *xscale = 0.f;
+    if (yscale)
+        *yscale = 0.f;
+
+    _GLFW_REQUIRE_INIT();
+    _glfwPlatformGetWindowContentScale(window, xscale, yscale);
+}
+
 GLFWAPI void glfwIconifyWindow(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;

src/wl_monitor.c

@@ -153,6 +153,15 @@ void _glfwPlatformGetMonitorPos(_GLFWmonitor* monitor, int* xpos, int* ypos)
         *ypos = monitor->wl.y;
 }
 
+void _glfwPlatformGetMonitorContentScale(_GLFWmonitor* monitor,
+                                         float* xscale, float* yscale)
+{
+    if (xscale)
+        *xscale = (float) monitor->wl.scale;
+    if (yscale)
+        *yscale = (float) monitor->wl.scale;
+}
+
 GLFWvidmode* _glfwPlatformGetVideoModes(_GLFWmonitor* monitor, int* found)
 {
     *found = monitor->modeCount;

src/wl_window.c

@@ -550,6 +550,15 @@ void _glfwPlatformGetWindowFrameSize(_GLFWwindow* window,
     // implemented, but for now just leave everything as 0.
 }
 
+void _glfwPlatformGetWindowContentScale(_GLFWwindow* window,
+                                        float* xscale, float* yscale)
+{
+    if (xscale)
+        *xscale = (float) window->wl.scale;
+    if (yscale)
+        *yscale = (float) window->wl.scale;
+}
+
 void _glfwPlatformIconifyWindow(_GLFWwindow* window)
 {
     // TODO: move to xdg_shell instead of wl_shell.

src/x11_init.c

@@ -743,6 +743,43 @@ static GLFWbool initExtensions(void)
     return GLFW_TRUE;
 }
 
+// Retrieve system content scale via folklore heuristics
+//
+static void getSystemContentScale(float* xscale, float* yscale)
+{
+    // NOTE: Default to the display-wide DPI as we don't currently have a policy
+    //       for which monitor a window is considered to be on
+    float xdpi = DisplayWidth(_glfw.x11.display, _glfw.x11.screen) *
+        25.4f / DisplayWidthMM(_glfw.x11.display, _glfw.x11.screen);
+    float ydpi = DisplayHeight(_glfw.x11.display, _glfw.x11.screen) *
+        25.4f / DisplayHeightMM(_glfw.x11.display, _glfw.x11.screen);
+
+    // NOTE: Basing the scale on Xft.dpi where available should provide the most
+    //       consistent user experience (matches Qt, Gtk, etc), although not
+    //       always the most accurate one
+    char* rms = XResourceManagerString(_glfw.x11.display);
+    if (rms)
+    {
+        XrmDatabase db = XrmGetStringDatabase(rms);
+        if (db)
+        {
+            XrmValue value;
+            char* type = NULL;
+
+            if (XrmGetResource(db, "Xft.dpi", "Xft.Dpi", &type, &value))
+            {
+                if (type && strcmp(type, "String") == 0)
+                    xdpi = ydpi = atof(value.addr);
+            }
+
+            XrmDestroyDatabase(db);
+        }
+    }
+
+    *xscale = xdpi / 96.f;
+    *yscale = ydpi / 96.f;
+}
+
 // Create a blank cursor for hidden and disabled cursor modes
 //
 static Cursor createHiddenCursor(void)
@@ -861,6 +898,7 @@ int _glfwPlatformInit(void)
 #endif
 
     XInitThreads();
+    XrmInitialize();
 
     _glfw.x11.display = XOpenDisplay(NULL);
     if (!_glfw.x11.display)
@@ -884,6 +922,8 @@ int _glfwPlatformInit(void)
     _glfw.x11.root = RootWindow(_glfw.x11.display, _glfw.x11.screen);
     _glfw.x11.context = XUniqueContext();
 
+    getSystemContentScale(&_glfw.x11.contentScaleX, &_glfw.x11.contentScaleY);
+
     if (!initExtensions())
         return GLFW_FALSE;
 

src/x11_monitor.c

@@ -338,6 +338,15 @@ void _glfwPlatformGetMonitorPos(_GLFWmonitor* monitor, int* xpos, int* ypos)
     }
 }
 
+void _glfwPlatformGetMonitorContentScale(_GLFWmonitor* monitor,
+                                         float* xscale, float* yscale)
+{
+    if (xscale)
+        *xscale = _glfw.x11.contentScaleX;
+    if (yscale)
+        *yscale = _glfw.x11.contentScaleY;
+}
+
 GLFWvidmode* _glfwPlatformGetVideoModes(_GLFWmonitor* monitor, int* count)
 {
     GLFWvidmode* result;

src/x11_platform.h

@@ -211,6 +211,8 @@ typedef struct _GLFWlibraryX11
     int             screen;
     Window          root;
 
+    // System content scale
+    float           contentScaleX, contentScaleY;
     // Helper window for IPC
     Window          helperWindowHandle;
     // Invisible cursor for hidden cursor mode

src/x11_window.c

@@ -2240,6 +2240,15 @@ void _glfwPlatformGetWindowFrameSize(_GLFWwindow* window,
         XFree(extents);
 }
 
+void _glfwPlatformGetWindowContentScale(_GLFWwindow* window,
+                                        float* xscale, float* yscale)
+{
+    if (xscale)
+        *xscale = _glfw.x11.contentScaleX;
+    if (yscale)
+        *yscale = _glfw.x11.contentScaleY;
+}
+
 void _glfwPlatformIconifyWindow(_GLFWwindow* window)
 {
     if (window->x11.overrideRedirect)

tests/monitors.c

@@ -92,21 +92,24 @@ static void key_callback(GLFWwindow* window, int key, int scancode, int action,
 
 static void list_modes(GLFWmonitor* monitor)
 {
-    int count, x, y, widthMM, heightMM, i;
+    int count, x, y, width_mm, height_mm, i;
+    float xscale, yscale;
     const GLFWvidmode* mode = glfwGetVideoMode(monitor);
     const GLFWvidmode* modes = glfwGetVideoModes(monitor, &count);
 
     glfwGetMonitorPos(monitor, &x, &y);
-    glfwGetMonitorPhysicalSize(monitor, &widthMM, &heightMM);
+    glfwGetMonitorPhysicalSize(monitor, &width_mm, &height_mm);
+    glfwGetMonitorContentScale(monitor, &xscale, &yscale);
 
     printf("Name: %s (%s)\n",
            glfwGetMonitorName(monitor),
            glfwGetPrimaryMonitor() == monitor ? "primary" : "secondary");
     printf("Current mode: %s\n", format_mode(mode));
     printf("Virtual position: %i %i\n", x, y);
+    printf("Content scale: %f %f\n", xscale, yscale);
 
     printf("Physical size: %i x %i mm (%0.2f dpi)\n",
-           widthMM, heightMM, mode->width * 25.4f / widthMM);
+           width_mm, height_mm, mode->width * 25.4f / width_mm);
 
     printf("Modes:\n");